Why do we sometimes see more than one shadow?

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Offline chris

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Why do we sometimes see more than one shadow?
« on: 11/10/2008 09:57:12 »
I was asked on the radio by someone yesterday why he saw two shadows of himself on a moonlit night. In the absence of any other source of light, can anyone think of a reason?
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lyner

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Why do we sometimes see more than one shadow?
« Reply #1 on: 11/10/2008 10:58:02 »
There could have been a reflection of the Moonlight from a light coloured surface nearby. You could find an explanation for it at the time by 'tracing back' from the secondary shadow and see where it comes from. You can get some pretty bright looking clouds on a moonlit night, for instance.
Vision is pretty non-linear at low light levels and it could be easy to see two shadows of the same apparent depth in moonlight when you would not spot the same effect if the Sun were in the same position.
« Last Edit: 11/10/2008 10:59:53 by sophiecentaur »

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Offline RD

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Why do we sometimes see more than one shadow?
« Reply #2 on: 11/10/2008 11:55:33 »
I was going to suggest zodiacal light as a source of the second shadow,
but looking at the exposure of this photo, moonlight is over 1000 times (ten stops) brighter,
so would not produce a noticeable shadow compared to that cast by the moon.

A reflection of moonlight, (e.g. from a window), or an artificial light source seem more likely explanations for a second shadow.
« Last Edit: 11/10/2008 12:04:59 by RD »

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Offline syhprum

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Why do we sometimes see more than one shadow?
« Reply #3 on: 11/10/2008 14:32:37 »
Under very clear skies it is possible to see shadows from the light provided by Venus.
I have observed this at Ularu and it is interesting that the edges are much much sharper due to the smaller size of the source
syhprum

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Offline LeeE

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Why do we sometimes see more than one shadow?
« Reply #4 on: 12/10/2008 15:53:15 »
He has double vision.
...And its claws are as big as cups, and for some reason it's got a tremendous fear of stamps! And Mrs Doyle was telling me it's got magnets on its tail, so if you're made out of metal it can attach itself to you! And instead of a mouth it's got four arses!

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Offline Soul Surfer

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Why do we sometimes see more than one shadow?
« Reply #5 on: 14/10/2008 18:49:08 »
Chris I think we need a little bit more information on the precise conditions were when this double shadow was observed.  ie location, angle between the shadows and if there were any reflecting surfaces peresent.  The double vision idea could be a possibility because object fixation can be difficult for some people in low light levels.
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Offline LeeE

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Why do we sometimes see more than one shadow?
« Reply #6 on: 14/10/2008 21:26:29 »
Yeah - it's possible that in the low-light conditions his eyes had diverged - just as you have to do when looking at those random-dot autostereograms.
...And its claws are as big as cups, and for some reason it's got a tremendous fear of stamps! And Mrs Doyle was telling me it's got magnets on its tail, so if you're made out of metal it can attach itself to you! And instead of a mouth it's got four arses!

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Offline techmind

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Why do we sometimes see more than one shadow?
« Reply #7 on: 15/10/2008 13:22:45 »
I've been amazed by how bright the (full) moon has been since the weekend. Where I used to live there was so much artificial light that I never really noticed the moonlight!
"It has been said that the primary function of schools is to impart enough facts to make children stop asking questions. Some, with whom the schools do not succeed, become scientists." - Schmidt-Nielsen "Memoirs of a curious scientist"

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Offline Alan McDougall

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Why do we sometimes see more than one shadow?
« Reply #8 on: 15/10/2008 15:36:07 »
Chris

The only other light source in a place with no light pollution at night etc is star light and that is much to faint to give a shadow.

I have been an amateur astronomer many years and have spent a lot of time out side in the dark and have never seen two shadows

I think a person who thinks he saw two shadows perceived an optical illusion

The retina is like an organic digital camera with two kinds of pixels: rods and cones. Cones allow us to see colors (red roses) and fine details (words in a book), but they only work in bright light. After sunset, the rods take over.

Rods are marvelously sensitive (1000 times more so than cones) and are responsible for our night vision. According to some reports, rods can detect as little as a single photon of light! There's only one drawback: rods are colorblind. Roses at night thus appear gray.

Alan
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